THE former fiancee of NBA star Jason Collins has blasted her ex for keeping her in the dark about his sexuality when he called off their wedding.

Carolyn Moos says she had no idea why the NBA veteran called off their engagement in 2009 until a Sports Illustrated article published in April named Collins as the first active athlete to come out as gay in the major sports.

In a piece titled "Jason Collins Is My Ex-Fiancé And I Had No Idea He Was Gay", Moos told US Cosmopolitan that she was frustrated the Washington Wizards player had ignored her since the Sports Illustrated story came out.

"A month before I was set to marry the man I loved, he called off the wedding. I had no idea why," Moos said in the August issue of magazine.

"He and I had been together for eight years. We had planned to have children, build a family. Nearly four years later, I got my answer.

"Jason told me he's gay over the phone on a Monday morning in April, the same day the magazine hit newsstands. However, he didn't mention the article - that came as a surprise when I heard about it from a friend."

NBA centre Jason Collins came out in April, becoming the first openly gay active player in major sports. Picture: Getty Images.

In July 2009 Collins had told Moos, a former USA basketballer, that he was "just not sure" about their relationship and cancelled their wedding.

"There were no tangible reasons, no explanations," she said.

"During all the years I had known him, I never would have guessed that he would come out as gay.

"I wish he could have been honest with me years ago. I feel like there are two Jason's now - the man I fell in love with and the man I'm trying so hard to understand. He's being hailed as a pioneer, but I believe true heroism is a result of being honest with yourself and with those you love."

Experts say finding out your partner is gay can feel like a double betrayal.

Lyn Fletcher from Relationships Australia said being cheated on with a member of the opposite sex was highly traumatic.

"It can be a huge shock to the other partner. They will be the ones who feel that they've been living a lie," she said.

"The person who's coming out often has inklings of that well before they come out so they've had time to adjust to it. The person on the receiving end hasn't.

"They might suspect it at times ... the same way that you might when your partner's having an affair but it depends on the age group and how aware they may be that it's even a possibility."

The relationship educator of 25 years said the discovery was like a "double whammy".

"It's a betrayal of trust and of the commitment in the relationship but it's also a betrayal of who you thought that person was," she said.

"It's a huge issue and goes to the heart of who you are as a person. It challenges your own identity. For men it challenges their virility.

"A lot of women think 'I failed him in some way and not given him what he needs'. They can sometimes feel they caused it in some way and they feel they're to blame for it. They need to recognise that they're never going to be able to satisfy them."

Wedding cake topper and flowers

While there is no Australian research to show how often heterosexual relationships break down due to homosexuality, Ms Fletcher said she had counselled countless couples where one partner was conflicted about their sexuality.

"I've counselled couples like this and I've had friends who've gone through it," she said.

"In my years of counselling I've come across it often enough that it doesn't make me bat an eyelid to talk about it or counsel somebody in that situation. It's common enough."

Ms Fletcher said she saw such separations more often in mature-aged couples.

"I think the likelihood for it to occur is greater in mature-aged couples than for younger people where being homosexual is far more socially acceptable than it was 30 years ago."

"Up until the 90s there was probably a certain percentage of the population who ... went into heterosexual relationships trying to suppress the part of themselves that was actually attracted to someone of the same sex."

Twenty10 managing director Rebecca Reynolds said their gay and lesbian counselling service commonly fielded calls from confused people in heterosexual relationships.

"They would have had mostly heterosexual relationships up until that point in time," she said.

"When someone starts questioning their sexuality and they're in a committed relationship, there's often a whole heap of love there. (The relationship) is usually built over many years of shared experiences and friendship and sexuality is just one part of that."

Ms Reynolds said there was still a lot of secrecy surrounding sexuality.

"The number of people who pick up the phone and ask for assistance is so small compared to those who don't talk or do anything about it due to that stigma," she said.

"There are feelings of guilt and betrayal towards their partner if they're in a heterosexual relationship but also towards themselves.

"We get a lot of phone calls from people who are questioning their sexuality and those who have someone say to them 'I'm attracted to someone of the same sex as me so I don't know what this means for our relationship'."

’I had no idea my fiance was gay’Source:Supplied

Ms Fletcher said there were sometimes signs that one partner was struggling with their sexuality.

"Some of the signs are things that might tell you someone's having an affair such as unexplained absences, being with one particular person a lot more and very close relationships with other people of the same sex which seem more than just friendships," she said.

"Everybody can (have fetishes) so I wouldn't say that's not necessarily a sign of homosexuality ... I've seen just as many heterosexual men come undone from (excessive porn use) as homosexual men.

"If the sex life within the relationship is not satisfying or stimulating (for the sexually conflicted partner) and you're thinking about what would make it better and the thing that comes to you is that you actually want to be with someone of my own sex, well there's your answer."

New York psychotherapist Jonathan Alpert said about 3 per cent of his clients were homosexuals living in heterosexual relationships.

He said they were mostly men aged from their late 20s to 50s.

"Denial is how they cope. They might come up with explanations such as: 'I was drunk' or 'it's just a one time occurrence' or 'it's just physical so it isn't cheating'.

"These gays clients are masterful at compartmentalisation. They are very skilled at separating various parts of their lives: husband, father, secret lover, etc.

"This behaviour is the epitome of selfishness. They're getting their needs met at the expense of their marriage, commitment and, of course, putting their partner's health in great danger."

The relationship expert and author of BE FEARLESS: Change Your Life in 28 Days said it was hard to estimate how many men or women in the general population are cheating in this fashion.

"I will say though, it is much more common than people think. I always warn my female friends to be careful because no matter how straight you think your man is, you never truly know what might be going on in his head and what he is up to when you're not together," he said.

"The signs and symptoms that this might be happening are similar to those of anyone stepping out of a marriage: the partner is anxious, secretive, seems stressed, and despondent.

"Also, if you see changes such as how he dresses or if he cares more about his appearance than he used to. These could all be signs that he is cheating in general."

Freelance journalist Kiri Blakely has written a book Can't Think Straight: A Memoir of Mixed-Up Love about her own Brokeback Mountain moment.

The former Forbes reporter found out her fiancé and partner of 10 years was gay and had been sleeping with men for at least two years before she found out.

"His betrayal left me more than devastated: It left me another person. I was now someone fearful and angry. I'd burst into spontaneous tears at work. I had panic attacks. I became convinced everyone was lying to me, even about the most benign things," she wrote in the New York Post.

"With time, I've gained more equilibrium (I hope). I'm even now in a relationship, though my trust issues certainly gave us some rocky moments. Aaron is now living with a man, and we still speak from time to time. I'm proud of him for leading an honest life."